Mesospheric Clouds on Mars in Nadir-Pointed THEMIS-VIS Images

We present images of very-high-altitude clouds obtained at near-zero emission angles by the Mars Odyssey spacecraft's Thermal Emission Spectrometer visible subsystem (THEMIS-VIS). Although THEMIS-VIS was not designed or intended for stereo imaging, the parallax effect caused by its multiple exposure color-imaging sequence allows the height of clouds with sufficient spatial contrast to be determined with a precision of 5-10 km. To determine cloud height in a given image, we make an initial guess of the height, reproject the subframes that make up the image onto a surface of that elevation, and then examine the reprojected imagine to determine whether the cloud features are properly aligned in overlapping subframes. This process is repeated until the best fitting elevation is found. The precision of the height estimate obtained in this manner is obviously limited by the sharpness of the cloud features. Although clouds and hazes are common in the THEMIS-VIS data set, images in which a cloud height can be measured are extremely rare. To date, we have only two detections of equatorial high-altitude clouds, both of which occur (possibly coincidentally) over high surface elevations in the eastern Tharsis bulge, but in quite different seasons --- {L} _{s} 26^o and {L} _{s} 114^o. In both cases, the detections are in late-afternoon images, near 4:30 PM local solar time, the cloud heights are near 75 km above the local surface, and the cloud structure is wispy and lineated. Our detections are consistent with limb observations by TES (solar band) and MOC (Wide Angle) of high altitude clouds (Clancy et al., 2004, DPS). Clancy et al.~find that these clouds appear to be present only within a very restricted range of longitudes and seasonal intervals. In addition to the equatorial cloud measurements, we have also in a few instances been able to measure heights of 60-70 km for mid-latitude clouds near the fringes of the polar hood in early northern winter. This measurement requires the serendipitous confluence of unusually distinct cloud features and favorable lighting conditions, and so, at this time, it is difficult to assess the prevalence or nature of high altitude clouds in this region and season.